Turbomachines generally have a centrally disposed rotor that rotates within a stationary cylinder or shell. The working fluid flows through one or more rows of circumferentially arranged rotating blades that extend radially from the periphery of the rotor shaft and one or more rows of circumferentially arranged stator blades that extend centripetally from the interior surface of the shell to the rotor shaft. The fluid imparts energy to the shaft that is used to drive a load, such as an electric generator or compressor. In order to ensure that as much energy as possible is extracted from the fluid, the tips of the stator blades are usually very close to the seals located on the rotor surface, and the tips of the rotating blades are usually very close to the seals located on the internal surface of the shell. From the standpoint of thermodynamic efficiency, it is desirable that the clearance between the stator blade tips and the seals on the rotor surface, and between the rotating blade tips and the seals on the shell be maintained at a minimum so as to prevent excessive amounts of fluid from bypassing the row of rotating blades and stator blades.
Unfortunately, differential thermal expansion during operating conditions between the shell and the rotor results in variations in the tip clearances. In addition various operating conditions affect tip clearances—for example, tip clearances in gas turbine compressors often reach their minimum values during shutdown, whereas the tip clearances in low pressure steam turbines often reach their minimum values at steady state full load operation. Consequently, if insufficient tip clearance is provided at assembly, impact between the stator blade tips and rotor seals and impact between the seals on the shell and the rotating blade tips may occur when certain operating conditions are reached. These impacts are commonly known as “rubs.” Also turbomachines are subjected to a variety of forces under various operating conditions, particularly during transient conditions, such as start-ups, shutdowns, and load changes. These forces may also cause rubs. Rubs often cause severe damage to the blades and seals of the turbomachine. However, in turbomachines with drum rotor type construction, space is limited and a large number of seals prevent the movement of individual seals to control the seal clearances. Accordingly, a method and apparatus for actively controlling the clearances in a turbomashine with a drum rotor type construction in order to prevent rubs is desired.